I like the second. Good capture. I prefer it over the other two because of the sky colour and the exposure of the city lights. The first picture's city lights aren't striking enough. For the last, I would prefer the sky to have some colour.
You've pretty much grasped the area of night shots pretty well. There is little I can nitpick about the shots and then again, those nitpicks don't really matter. IMO the second shot is close to perfect already and doesn't really need post-processing.
You may want to do minor things like increasing saturation or USM but it's not fantastically necessary. The tough thing about night shots is exposure and you've nailed it, well done.